1. Field of Invention
The present invention relates to methods and apparatus for generating low power density glow discharge plasmas at atmospheric pressure.
2. Description of the Prior Art
In the discipline of physics, the term "plasma" describes a partially ionized gas composed of ions, electrons and neutral species. This state of matter may be produced by the action of either very high temperatures, strong constant electric fields or radio frequency (R.F.) electromagnetic fields. High temperature or "hot" plasmas are represented by celestial light bodies, nuclear explosions and electric arcs. Glow discharge plasmas are produced by free electrons which are energized by imposed direct current (DC) or R.F. electric fields, and then collide with neutral molecules. These neutral molecule collisions transfer energy to the molecules and form a variety of active species which may include photons, metastables, atomic species, free radicals, molecular fragments, monomers, electrons and ions. These active species are chemically active and/or physically modify the surface of materials and may therefore serve as the basis of new surface properties of chemical compounds and property modifications of existing compounds.
Very low power plasmas known as dark discharge coronas have been widely used in the surface treatment of thermally sensitive materials such as paper, wool and synthetic polymers such as polyethylene, polypropylene, polyolefin, nylon and poly(ethylene terephthalate). Because of their relatively low energy content, corona discharge plasmas can alter the properties of a material surface without damaging the surface.
Glow discharge plasmas represent another type of relatively low power density plasma useful for non-destructive material surface modification. These glow discharge plasmas can produce useful amounts of visible and ultraviolet radiation. Glow discharge plasmas have the additional advantage therefore of producing visible and UV radiation in the simultaneous presence of active species. However, glow discharge plasmas have heretofore been successfully generated typically in low pressure or partial vacuum environments below 10 torr, necessitating batch processing and the use of expensive vacuum systems.
Conversely, the generation of low power density plasmas at one atmosphere is not entirely new. Filamentary discharges between parallel plates in air at one atmosphere have been used in Europe to generate ozone in large quantities for the treatment of public water supplies since the late 19th century. Filamentary discharge plasmas are dramatically distinguished from uniform glow discharge plasmas. Such filamentary discharges, while useful for ozone production, are of limited utility for the surface treatment of materials, since the plasma filaments tend to puncture or treat the surface unevenly.
It is, therefore, an object of the present invention to teach the construction and operating parameters of a continuous glow discharge plasma sustained at a gas pressure of about one atmosphere or slightly greater.